1. Field of the Invention
This invention, in general, relates to optical systems and, in particular, to a convertible type of photographic objective that is separable into two working parts consisting of a front part that can be rotated or translated laterally into or away from a coaxial position with a rear part which normally is fixed in position on an assigned optical axis.
2. Description of the Prior Art
For hand photography, considerations of cost and convenience are paramount as are simplicity and reliability of operation. It is also desirable that a camera intended for use both indoors and outdoors by a relatively untrained individual be light weight and adapted for carrying or for safe storage when not in use, as on a shelf or in a drawer. These several requirements are commonly met in the modern era by the use of molded plastic mounts, lenses and body parts of the camera, inasmuch as the specific gravity of plastic is low in comparison with ordinary glass and metallic components. In particular, for cameras of the class required for instant photography, the arrangement of the camera loading and print ejection must be considered, along with the shape and weight limitations indicated above.
It is also desirable with respect to a camera to be used by the general public for instant photography that the single focal length supplied in inexpensive non-zooming hand cameras be augmented or replaced by the use of at least two significantly different focal lengths for the convenience and enjoyment of the photographer. Thus, for closeups and for group pictures requiring a fairly large total angle of view, as for example, a picture diagonal of 50 degrees more or less, one needs to have an equivalent focal length of the order of 115 mm for picture formats of usable size as practiced in the instant photographic field, commonly 3 inches square more or less. Conversely, for portraits and for more distant subject matter one normally wishes to employ an equivalent focal length of greater magnitude, say, in the range from 135 mm to 150 mm, more or less.
To fulfill these requirements one could, of course, provide a camera having two complete objectives of focal lengths 115 mm and 150 mm, for example, each with iris and shutter and suitable electronic and hand operated controls. Considerations of space and cost call instead for some form of less costly convertible objective having either focal length, according to a hand operated or electrically operated control, but occupying essentially the same space within the camera insofar as a single optical axis, single iris and single shutter are concerned.
In the earlier days of photography, so-called convertible objectives were in common use. Then, one screwed on a front part of the objective to obtain a shorter focal length, or used either the front part or the rear part separately to obtain a longer focal length. The fully assembled convertible objective with its shorter equivalent focal length generally was a symmetrical or nearly symmetrical objective with the front and rear parts opposed and curved generally around a central iris or aperture stop and "between-the-lens" shutter. Objectives of those earlier years were almost always composed of various kinds of optical glasses and contained from one to as many as five cemented elements in either of the front and rear components. Some convertible objectives departed quite widely from symmetry, and combination of "new" and "old" glass types were used in the front and rear halves to obtain special corrections.
Convertible objectives of the early form are not well suited to the requirements of a hand camera used for instant photography. Glass lens elements and all-glass objectives are quite heavy and in any event too costly, inasmuch as each element with its two surfaces must generally be ground and polished to prescription, either individually or in blocks. The high curvatures of the strongest elements often require one-at-a-time fabrication of at least one of the two surfaces. Moreover, it is inconvenient to unscrew a front component or to remove it from a bayonet-type mount and to carry it separately from the camera. Such handling of plastic components leads to a progressive deterioration of the lens elements as well. In any event, interchangeable parts of cameras carried separately are not well suited to amateur use in family activities.
Consequently, the ability to extend the focal length of a camera objective lens with a magnifying attachment without changing the effective aperture ratio of the objective is particularly desirable. One attachment commonly proposed for this purpose is a form of Galilean telescope. The Galilean telescope basically comprises a convergent, or positive, component followed by a divergent, or negative, component which, together, form an afocal system, i.e., the object and image planes are at infinity, as are the conjugate focal points. Such a lens condenses a parallel bundle of rays into a smaller parallel bundle effecting a magnification, when the system is focused through a following objective, that depends upon the ratio of the diameters of the entrant and emergent beams. Such systems have long been known as image magnifying attachments for increasing the effective focal length of a lens.
Typically, a Galilean system suffers from field curvature and distortion, and has a narrow field angle, resulting in appreciable vignetting when used with a camera lens having a normal field of view; for example, 40 to 50 degrees. The problem of designing and correcting such a lens is compounded by the location of the camera lens entrance pupil, which becomes the exit pupil for the afocal attachment, well back in the camera. The result is a highly asymmetrical system when the attachment is in place, whereas it would be most desirable to have the limiting stop symmetrically located.
Efforts to improve afocal systems have in general had as objectives either to simplify the problem of correcting for aberrations or to achieve a more compact construction than the simple Galilean telephoto system provides.
As an example of the first approach, U.S. Pat. No. 3,094,580, issued on June 18, 1963 to R. Rosier, describes an afocal system of the concentric type. In the practical embodiment described by Rosier, there are six elements in two air-spaced components. In such a system, the converging and diverging surfaces of the principal parts of the lens are concentric, which considerably simplifies the correction of a number of the significant aberrations. This approach is quite appropriate for use in making a telephoto attachment for small cameras, such as 8 mm or 16 mm movie cameras, for example. However, because the physical size of a lens constructed on the concentric principle tends to be large compared to the focal length of the objective with which it is to be used and because relatively large amounts of expensive glasses are necessarily incorporated in the design, such a lens is not particularly well suited to use with the larger formats.
With six elements, and by the use of a relatively large air space between the front positive and rear negative group of components, I have found it possible to construct a compact afocal system that can be well corrected for any particular objective. Such six element lenses are described in U.S. Pat. No. 3,604,786, issued to me on Aug. 14, 1971, for "Compact Afocal Magnifying Lens".
More recently in U.S. Pat. No. 4,171,872 I have described compact four element afocal attachments which utilize a strongly curved first surface followed by other strongly curved surfaces, using alternating high and low indices of refraction for the successive negative and positive elements, and by correcting the lens less well for longitudinal color and distortion than for other aberrations.
Five element afocal systems that are reasonably well corrected, with the exception of marginal performance with respect to distortion and field curvature, have been designed for relatively smaller magnifications. For example, the Yashinon 1.4.times.afocal attachment, as made commercially by the Sun Optical Company of Osaka, Japan is such a lens system.
The principal objects of my invention are to simplify the construction, and to decrease the size and weight of afocal attachments for use with objective lenses having buried stops, while retaining acceptable levels of performance, particularly in the field of medium format photography; i.e., for use with film formats having diagonals from 3 to 6 inches more or less.
Another object of my invention is to provide a compact, afocal attachment that is reasonably well corrected for use in medium format photography at modest relative apertures, say no faster than f/10.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed description.